Galaxy cakes have exploded on Instagram, but making the mirror glaze on top can be tricky. We created this easy version using store-bought frosting instead. Place a round layer cake on an upside-down cake pan set on a baking sheet and cover with a thin, even layer of white frosting. Tint 2 cups frosting dark blue using gel food coloring, then tint another 1/2 cup teal, 1/2 cup purple and 1/2 cup royal blue. Microwave the dark blue frosting in 30-second intervals until it's very runny, then repeat with the other colors. Working quickly, pour the dark blue frosting over the cake to cover completely, letting it drip down the side. Drizzle a few lines of the other colors over the cake, then smooth over them with an offset spatula. Using a small pastry brush, splatter the top with white food coloring, sprinkle with edible glitter and let set.